An industry standard requirement for four-pair communication cable defined (as of the filing date of this application) in standards from the Telecommunications Industry Association/Electronic Industries Alliance, known as TIA/EIA-568, allows a propagation delay (ns/100 m) for a cable to be no greater, for example, than 534+(36/√f) ns/100 m or 534 plus 36 divided by the square root of the frequency, expressed in nanoseconds per 100 meters of cable length. Computation of propagation delay is based on the velocity of propagation of a signal from one end of a conductor pair of a cable to the other end of the conductor pair of the cable and is related at least in part to the insulation material and the surrounding materials in the cable. Thus, if only air and the surrounding insulation are present in the communication cable, the signal propagation proceeds at a very high velocity. However, if oil or water or some other material is present surrounding the insulation, the velocity is considerably less.
Cables that are designed for outdoor applications require the incorporation of a water-blocking material between and around the conductor pairs to prevent water from entering the cable through the ends or through a damaged area such as a cut or tear in the outer jacket. Traditional water-blocking compounds greatly reduce the speed of the electrical signal, i.e., the velocity of signal propagation, through the cable and therefore cause excess delay between the time the signal is sent and the time it is received at the other end (i.e., propagation delay).
In order to achieve an acceptable velocity of propagation or propagation delay it has heretofore been necessary to use foamed insulation. The foaming is accomplished by incorporating small gas bubbles into the insulation matrix to reduce the dielectric constant of the insulation. However, not only does the presence of these bubbles greatly weaken and reduce the tensile strength of the insulation, it also makes the insulation less resistant to size distortion from crushing or compression and less resistant to tearing, scratching and cut-through. Further, the industry standard propagation requirement is not achievable with currently available conventional filling compounds and solid (non-foamed) insulation.
In addition, when communication cable is laid in long vertical runs, for example, up towers such as cell phone towers, if the viscosity is too low, a drip-wise loss of the filling compound can occur. Such dripping of the filling compound not only results in loss of the water blocking benefits imparted by the filling compound to the communications cable but can also cause fouling of equipment and components upon which dripping of the filling compound occurs.
Communication cable employing existing art filling compounds typically have a higher density and a lower molecular weight that translates, for example, into a lower viscosity at operating temperatures, and all experience dripping or running of the compound in such vertical installations. Commercially available non-drip filling compounds formulated specifically as a water blocking agent for communication cable, such as extended thermoplastic rubber (ETPR) or polyethylene modified petroleum jelly (PEPJ) have proven unsuitable because the resulting cable does not meet industry standards for propagation delay.
Some current designs have incorporated a barrier layer of polyethylene terephthalate (PET) to contain the compound within its contents. Another attempt to address the dripping problem is the use of an inner jacket of a material such as Mylar™ PET tape to retain the filling compound. Such attempts have been only partially successful in that the oil component of the filling compound seeps or migrates through or around the barrier layer or jacket over time. A related attempt to address the problem is use of a Mylar™ inner core wrap, which is likewise only partially successful and even more expensive.
Accordingly, to address these representative deficiencies in the art, what is needed is an improved capability to allow for an acceptable signal propagation delay or velocity of propagation in water blocked communication cable as required by communication cable industry specifications. Another need exists for a water blocked communication cable with an increased velocity of propagation that allows for the use of solid insulation of the conductors. A further need exists for a water blocked communication cable with such a filling compound with anti-dripping characteristics in vertical installations. A still further need exists for a water blocked communication cable with such a filling compound that significantly reduces the amount of material that is absorbed through the communication cable jacket.
A capability addressing one or more of these needs would significantly decrease the cost of making and using and significantly improve the performance of water blocked communication cable.